1. Field of the Invention
The present invention generally relates to an apparatus for and a method of detecting a mode of an image signal, and more particularly, to an apparatus for, and a method of detecting whether the image signal is from a film source, by generating and accumulating a pattern using motion information of the image signal and then by comparing the accumulated pattern with a threshold.
2. Description of the Related Art
Generally, a film source, such as a movie, includes 24 progressive frames per second. As for the National Television System Committee (NTSC) TV signal and DVD signal, for example, each signal includes 60 interlaced fields per second. Accordingly, in order to broadcast the film source to a TV set (terminal), the 24 progressive frames per second need to be converted to the 60 interlaced fields per second. It is a 3:2 pull-down scheme (conversion) that is used in the above conversion.
The 3:2 pull-down scheme (conversion) converts two 24 Hz frames into five 60 Hz fields. More specifically, one field is repeated to form three fields from a first frame while two fields are formed from a second frame. The 3:2 pull-down scheme extracts from the first frame a top field having odd lines and a bottom field having even-lines, and then re-extracts (repeats) the top field. After that, the 3:2 pull-down scheme extracts another bottom field and another top field from the second frame.
Using the 3:2 pull-down scheme, the two progressive frames are converted into the five interlaced fields. Accordingly, as the film source is 3:2 pull-down converted and transmitted from a transmitting end (terminal or station), a receiving end (terminal or station) obtains the complete original progressive frames before the 3:2 pull-down conversion, through a combination of the two fields of the 3:2 pull-down converted interlaced fields. In other words, the receiving terminal can obtain original images having a high quality according to an Interlace-to-Progressive Conversion (IPC) process and the 3:2 pull-down converted interlaced fields.
Accordingly, the receiving terminal needs to carry out detecting of a film mode whether a received image is from a 3:2 pull-down converted film source or from a non-film source, and then processing image signals of the received image accordingly. Such film mode detection and image signal processing are also required for the image signals other than the NTSC TV signal.
FIG. 1 is a block diagram showing a conventional apparatus for detecting a film mode of an NTSC image signal. A conventional film mode detecting scheme calculates a sum of absolute difference (SAD) of two neighboring fields of a 3:2 pull-down converted image signal, and then detects the film mode of the image signal using a period of 5 SADs.
As shown in FIG. 1, the conventional film mode detecting apparatus includes a field buffer 10, a difference calculator 18, an absolute value calculator 20, a field adder 30, a limiter 40, a band-pass filter 50, a power calculator 60 and a mode detector 70.
The field buffer 10 stores fields of incoming image signals. The field buffer 10 includes a first field buffer 12, a second field buffer 14 and a third field buffer 16. Accordingly, the fields of the incoming image signals are sequentially stored in and outputted from the first, the second and the third buffers 12, 14 and 16.
The difference calculator 18, the absolute value calculator 20 and the field adder 30 calculate the SAD of the image signal. The difference calculator 18 calculates a difference of the respective pixels allocated in the same position of the respective fields that are stored in the first and the third buffers 12 and 16. The absolute value calculator 20 calculates an absolute value of the difference of the pixels of the respective fields calculated at the difference calculator 18.
The field adder 30 adds the absolute value of the difference of the pixels calculated at the absolute calculator 20 in unit of field. Accordingly, SADs with respect to the respective fields of the incoming image signals are calculated.
Among the SADs calculated with respect to the respective fields, the limiter 40 limits an amplified value of the SAD, which can cause a scene change, i.e., a main cause of defective detection of the film mode, to a predetermined threshold.
The band-pass filter 50 band-pass filters the SADs transmitted from the limiter 40, thereby extracting signals of 5-field period with respect to a frequency axis. The signals with 5-field period, which are extracted from the band-pass filter 50, are sinusoidal waves.
The power calculator 60 calculates the power of sinusoidal waves outputted from the band-pass filter 50. Accordingly, through such calculated power, it can be checked how much of the SADs of the respective fields have a period of the 5 fields.
The mode detector 70 compares the predetermined threshold and the power calculated at the power calculator 60, thereby determining the film mode of the incoming image signals based on a comparison result. That is, the conventional mode detector 70 determines the incoming image signals are in the film mode when the power calculated at the power calculator 60 is greater than the predetermined threshold. The conventional mode detector 70 determines the incoming image signals in the non-film mode when the power calculated at the power calculator 60 is less than the predetermined threshold. Also, according to the comparison result between he predetermined threshold and the power calculator 60, the conventional mode detector 70 outputs ‘1’ for the film mode or ‘0’ for the non-film mode together with the incoming image signals to an IPC unit that performs the IPC process, or to a storage unit that stores the incoming image signals.
FIG. 2 is a graph showing the SADs calculated with respect to the respective fields that are outputted from the field adder 30 of FIG. 1. Here, it is assumed that no noise has occurred during transmitting the 3:2 pull-down converted image signals (streams).
Referring to FIG. 2, the scene change occurs at a point of field no. 9, which prevents finding a period of the 5 fields. Accordingly, the limiter 40 limits the amplified value of the SAD at the field no. 9 to the predetermined threshold, thereby eliminating the cause of a scene change. As the band-pass filter 50 band-pass filters the limited SAD with a center at ω=2π/5 and a DC gain at ‘0’, the outputted SAD is a sinusoidal wave having a period of ⅕ field with respect to the time axis. As a property of the SAD having ⅕ period increases, a relatively large amplitude of the sinusoidal wave is obtained. In order to calculate the power with respect to the amplitude of the sinusoidal wave, the power calculator 60 calculates a sum of squares of the amplitude, to thereby determine whether the period of the SADs is the 5 fields. Accordingly, after comparing the SAD with the predetermined threshold, the mode detector 70 determines the incoming image signals are in the film mode when the SAD exceeds the predetermined threshold, and then outputs a determination result.
Usually, the SADs of the two fields of the 3:2 pull-down converted stream (incoming image signal) have the period of the 5 fields. However, such periodicity of the SADs causes a loss of regularity of the SADs when more noise is added to the SADs.
Another problem is that an incorrect periodicity of the incoming image signals can be outputted. That is, when the limiter 40 limits a peak value of the SAD to eliminate the cause of the scene change, the limiter 40 removes a value of the SAD by the predetermined value depending on the 3:2 full-down converted stream even when the value of the SAD is small compared to the predetermined value.
Further, the mode detector 70 has the predetermined threshold to determine the film mode of the incoming image signals based on the comparison of the SADs. Here, since incoming streams can have different powers, using a fixed threshold for all the incoming streams can cause unreliable film mode detection of the image signals.
In order to overcome the above problem, conventionally, many candidates for the thresholds have been used to obtain appropriate thresholds. However, with a lot of noise in the incoming stream and great SAD changes between the neighboring fields, the film mode detection can be quite inaccurate.